The present invention relates to evaporative control systems for hybrid and non-hybrid vehicles, and more specifically to an evaporative canister system that reduces breakthrough.
Gasoline typically includes a mixture of hydrocarbons ranging from high volatility butane (C-4) to lower volatility C-8 to C-10 hydrocarbons. When vapor pressure increases in the fuel tank due to conditions such as ambient temperature, fuel vapor flows through openings in the fuel tank. To prevent fuel vapor loss into the atmosphere, the fuel tank is vented into a canister that contains an absorbent material such as activated carbon granules.
As the fuel vapor enters an inlet of the canister, the fuel vapor diffuses into the carbon granules and is temporarily adsorbed. The size of the canister and the volume of the adsorbent material are selected to accommodate the expected fuel vapor evaporation. After the engine is started, the control system uses engine intake vacuum to draw air through the adsorbent to desorb the fuel. The desorbed fuel vapor is directed into an air induction system of the engine as a secondary air/fuel mixture. One exemplary evaporative control system is described in U.S. Pat. No. 6,279,548 to Reddy, which is hereby incorporated by reference.
When the vehicle remains idle, fuel vapor accumulates in the canister. The initial loading is at the inlet end of the canister. Over time, the fuel vapor is gradually distributed along the entire bed of the adsorbent material. After the engine is started, a purge valve is opened and air is drawn through the canister. The air removes the fuel vapor that is stored in the adsorbent material.
An evaporative control system according to the present invention for a vehicle includes a fuel tank for storing a volatile fuel and an engine having an air induction system. A primary canister contains a first volume of a first adsorbent material, a vapor inlet coupled to the fuel tank, a purge outlet coupled to the air induction system, and a vent/air inlet. A secondary canister is coupled to the vent/air inlet and contains a second volume of a second adsorbent material that is different than the first adsorbent material. The first and second adsorbent materials adsorb fuel vapors when the engine is not running to reduce breakthrough and desorb fuel vapors when the engine is running.
In still other features, the second adsorbent material may include activated carbon derived from a coconut shell. The first adsorbent material may include activated carbon derived from wood. In certain embodiments, the evaporative control system may reduce breakthrough below 4 mg/day.
In yet other features, the secondary canister includes a housing and a heater that heats the secondary volume of the second adsorbent material. Alternately, the secondary canister includes a housing, a heater located outside of the housing and a heat sink. The heater heats the heat sink. The heat sink heats the secondary volume of the second adsorbent material. The heat sink includes a plurality of plates that are coated with the second adsorbent material.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.